1. Field of the Invention
This invention relates to a process for the production of ethylene polymers and more particularly to a process for producing polyethylene of ultrahigh molecular weight by employing a selected mode of multi-stage polymerization reaction and a catalyst of a selected composition therefor.
2. Description of the Prior Art
An ethylene polymer with an extremely high molecular weight of about one million or greater is generally known as ultrahigh molecular weight polyethylene (hereinafter referred to simply as "UHMW polyethylene"). UHMW polyethylene is known in common use as engineering plastic material characterized by high impact and wear resistance and self-lubricity, and hence it has found extensive application to hoppers, silos, gears, linings and the like for use in the various fields of industry including food processing, civil engineering, chemistry, agriculture and mining, and further to backings for skis, and sports and leisure supplies.
Because of its high molecular weight, UHMW polyethylene is literally too viscous for molding in hot melt and even in solution, meaning that it is only marginally moldable. This presents a bottleneck in the efforts of opening up new areas of market for UHMW polyethylene in spite of the aforesaid excellent properties.
Methods commonly used to mold UHMW polyethylene are compression, extrusion, injection, spinning, casting and the like. The molding or forming operation of UHMW polyethylene was very slow and involved extremely high pressure and other stringent conditions, and was capable of forming articles of only limited dimensions and poor finish. UHMW polyethylene was thus considered economically infeasible compared to ethylene polymers of lower molecular weights of about 10,000 to about 200,000. In solution moldings such as spinning and casting where UHMW polyethylene material is dissolved in organic solvents, the material was difficult to solubilize as fast and homogeneously as desired.
Alternatively, polyolefin waxes and petroleum resins have been used as additives to improve moldability of UHMW polyethylene, but their compatibility was not satisfactory, resulting in molded articles of reduced mechanical strength and low impact resistance.